Many different methods for the preparation of epoxides have been developed. Commercially, propylene oxide is produced by the chlorohydrin process or hydroperoxidation (see, e.g., U.S. Pat. Nos. 3,351,635 and 4,367,342; EP 0 345 856). Unfortunately, both processes have disadvantages. The chlorohydrin process suffers from the production of a dilute salt stream. The hydroperoxidation process, in which propylene is oxidized with an organic hydroperoxide such as ethylbenzene hydroperoxide or tert-butyl hydroperoxide, produces organic co-products such as t-butyl alcohol or styrene, whose value must be captured in the market place. Ethylene oxide is commercially produced by the direct oxidation of ethylene with oxygen over a silver catalyst. Unfortunately, efforts to epoxidize higher olefins (olefins containing three or more carbons) such as propylene with oxygen in the presence of a silver catalyst have failed to produce a commercial process (see, e.g., U.S. Pat. Nos. 5,856,534, 5,780,657 and 4,994,589).
Recent efforts have focused on the direct epoxidation of higher olefins with oxygen and hydrogen. For example, the reaction may be performed in the presence of a catalyst comprising gold and a titanium-containing carrier (see, e.g., U.S. Pat. Nos. 5,623,090, 6,362,349, and 6,646,142), or a catalyst containing palladium and a titanium zeolite (see, e.g., JP 4-352771).
Mixed catalyst systems for olefin epoxidation with hydrogen and oxygen have also been disclosed. For example, Example 13 of JP 4-352771 describes the use of a mixture of titanosilicate and Pd-on-carbon for propylene epoxidation. U.S. Pat. No. 6,008,388 describes a catalyst comprising a noble metal and a titanium or vanadium zeolite, but additionally teaches that the Pd can be incorporated into a carrier before mixing with the zeolite. The catalyst carriers disclosed include silica, alumina, and activated carbon.
Unfortunately, undesirable reactions also occur in these epoxidation processes. For example, the produced epoxide tends to react with solvents (e.g., water, methanol) to form glycols and/or glycol ethers. These side reactions are catalyzed by acids, which are formed as byproducts during the epoxidation. U.S. Pat. No. 6,498,259 discloses the epoxidation of an olefin with hydrogen and oxygen in a solvent containing a buffer in the presence of a catalyst mixture containing a titanium zeolite and a noble metal catalyst. Although a buffer can reduce the formation of the byproducts, it tends to complicate product purification. For example, a buffer can cause the fouling of distillation columns due to salt formation.
In sum, new processes for olefin epoxidation with reduced byproduct formation are needed.